1. Field of the Invention
The present invention relates to an image capturing apparatus and image capturing apparatus control method.
2. Description of the Related Art
Some image capturing apparatuses such as digital cameras and digital video cameras have both a function of performing moving image shooting and a function of performing still image shooting. In the image sensing apparatus, an image sensor such as a CCD sensor or CMOS sensor includes a pixel array in which a plurality of pixels including photoelectric conversion units are arrayed in the row and column directions, and a readout circuit for reading out a signal from the pixel array.
According to a technique disclosed in Japanese Patent Laid-Open No. 2001-352483, an image capturing apparatus switches the readout circuit driving method between successive frame periods from the first driving method suitable for moving image shooting to the second driving method suitable for still image shooting. According to the first driving method, the readout circuit is driven to read out signals from the pixel array at a low pixel density suitable for moving image shooting. According to the second driving method, the readout circuit is driven to read out signals from the pixel array at a high pixel density suitable for still image shooting. According to Japanese Patent Laid-Open No. 2001-352483, continuous shooting between moving image shooting and still image shooting can be achieved.
The technique in Japanese Patent Laid-Open No. 2001-352483 is premised on that no moving image shooting is performed during still image shooting.
However, some image capturing apparatuses provide a mode (to be referred to as a first mode hereinafter) in which only moving image shooting is performed, and a mode (to be referred to as a second mode hereinafter) in which parallel shooting of still and moving images is performedmoving image shooting.
In the first mode, the image capturing apparatus performs an operation suitable for moving image shooting. More specifically, the image capturing apparatus drives the readout circuit to read out signals from the pixel array at a pixel density decreased by addition, decimation, extraction, or the like, so as to read out signals in a readout period shorter than a frame period suitable for moving image shooting.
In the second mode, the image capturing apparatus performs an operation which gives priority to still image shooting. More specifically, the image capturing apparatus drives the readout circuit to read out signals in a readout period longer than a frame period suitable for moving image shooting, so as to read out signals from the pixel array at a high pixel density suitable for still image shooting. Also, the image capturing apparatus controls each pixel of the pixel array to accumulate a signal in an accumulation period shorter than one suitable for moving image shooting so that each pixel of the pixel array accumulates a signal at a shutter speed, i.e., in an accumulation period suitable for still image shooting. In still image shooting, the shutter speed, i.e., accumulation period needs to be set shorter than an accumulation period (accumulation period in the first mode) suitable for moving image shooting in order to prevent blurring of an image upon the movement of an object.
In the image capturing apparatus, the mode of the image capturing apparatus is sometimes switched from the first mode to the second mode between successive frames. In this case, if the image capturing apparatus maintains a frame rate suitable for moving image shooting, the readout period in the second mode becomes longer than a 1-frame period, and a drop frame of an obtained moving image may occur.
For example, as shown in FIG. 6, the image capturing apparatus keeps the lengths of frame periods FT1 to FT9 constant to achieve a frame rate suitable for moving image shooting regardless of the mode. The image capturing apparatus switches from the first mode to the second mode between the frame periods FT1 and FT2. The image capturing apparatus switches from the second mode to the first mode between the frame periods FT2 and FT3. The image capturing apparatus switches from the first mode to the second mode between the frame periods FT3 and FT4. The image capturing apparatus switches from the second mode to the first mode between the frame periods FT4 and FT5.
In FIG. 6, VD represents a frame sync signal (pulse), and is generated at an interval of 1/60 sec for an NTSC standard signal. The readout operation, moving image generation operation, and still image generation operation start at timings synchronized with the frame sync signal VD.
A, C, E, F, G, H, and I represent signals for moving images, and B and D represent signals for still images. First accumulation periods Tsa_A, Tsa_C, Tsa_E, Tsa_F, Tsa_G, Tsa_H, and Tsa_I in which the photoelectric conversion unit executes an accumulation operation in the first mode are fixed to 1/60 sec. Second accumulation periods Tsa_B and Tsa_D in which the photoelectric conversion unit executes an accumulation operation in the second mode are shorter than the first accumulation period and are shorter than 1/60 sec.
First readout periods Tsr_A, Tsr_F, Tsr_G, Tsr_H, and Tsr_I in which the readout circuit executes a readout operation in the first mode are shorter than 1/60 sec in accordance with the readout clock rate and the number of output channels. The first readout period is shorter than each of the frame periods FT1 to FT9. Second readout periods Tsr_B and Tsr_D in which the readout circuit executes a readout operation in the second mode are longer than the frame periods FT1 to FT9. Since the second readout periods Tsr_B and Tsr_D are longer than the frame periods FT1 to FT9, the readout circuit cannot start the readout operations of the signals C and E at timings synchronized with the frame sync signal VD.
In the moving image generation operation, the signals A, B, D, F, G, and H are used to generate image signals for 1-frame moving image (Tmv_A, Tmv_B, Tmv_D, Tmv_F, Tmv_G, and Tmv_H). As described above, the readout circuit cannot perform the readout operations of the signals C and E, so image signals for moving images corresponding to the signals C and E cannot be generated. As a result, drop frame of frames corresponding to the signals C and E would occur.
In the still image generation operation, the signals B and D are used to generate image signals for 1-frame still images.